1. Field of the Invention
The present invention generally relates to digital audio information processing, and particularly, to serial digital video interface audio sample processing, and more particularly, to improvement of an audio channel first-in first-out (FIFO) memory buffer enhancing audio signal quality and maintaining synchronization with video information.
2. Description of the Related Art
A digital audio or video recording is no more than a series of numbers, and hence can be copied through an indefinite number of generations without degradation. This implies that the life of a recording can be truly indefinite, because even if the medium begins to decay physically the sample values can be copied to a new medium with no loss of information. For the highest-quality post-production work, digital interconnection between such items as switchers, recorders and effects machines is highly desirable to avoid the degradation due to repeated conversion and filtering stages.
Digitized audio and video information, although sampled at different rates, can be multiplexed to provide cabling savings. A distinct advantage of such multiplexed transmission is where numerous pieces of video/audio equipment need to be interconnected in various ways for different purposes such as in a professional studio environment, a matrix distribution unit is more easily realized than that of having separate video and audio connections. A number of standards has been or is being developed to establish guidelines for serial digital video interface in between studio equipment which accomodates the video and audio information multiplex for data transmission, such standards are the International Radio Consultative Committee (CCIR) Recommendations 656&601-2, and the proposed Society of Motion Picture and Television Engineers (SMPTE) Standard 125M. In general, in transmitting digital video images with a 27 MHz transmission frequency, a line of video information is separated from another by a horizontal sync interval which provides a sufficient amount of time allowing the electron beam steering mechanism to adjust from the end of one raster line to the beginning of the next raster line. Ancillary information such as digitized audio samples each having binary values and a specified word length may be multiplexed into these sync intervals. Therefore, audio information associated with a video frame image may be embeded between lines of that video frame.
Despite the ease of transmission routing in between pieces of equipment, i.e., carrying video and audio information on one cable instead of two, however, within each equipment the combined video and audio signal often requires demultiplexing in order to meet separate processing demands. Since the audio information inserted into the sync intervals is typically digitized at a sampling frequency different from 27 MHz (the transmission frequency), for instance at 48 KHz, a FIFO memory buffer acting as an audio data receiver is usually necessary to receive the extracted audio data at the transmission frequency, 27 MHz, and to output audio data at the original sampling frequency, namely, 48 KHz, ensuring synchronization with its respective video data thereby.
Typically, there is a fixed number of audio samples per video frame, for instance, in accordance to the aforementioned standards, there are 8008 audio samples for every 5 video frames in a 525 line/59.94 Hz system. In this case, the number of audio samples in the FIFO buffer is maintained at a particular value for every 5 video fields, meaning that each audio sample stays in the FIFO buffer for approximately the same amount of time. If any additional time delay or time advance relative to the video information is introduced at the audio FIFO memory buffer stage, video and audio information would be out of sync. There are various reasons for such an additional delay or advance, one of which is that the FIFO memory buffer output frequency drifts against its input frequency, for example, processing images made by equipment not having the video and audio frequencies phase locked. Another of which is the extensive signal switching activities. Because the total number of audio samples of any one video field may not be exactly the same as its immediate successor field, switching activities such as generating image composites from various sources may cause a gradual increase or decrease of audio samples in the FIFO buffer. If uncorrected when recorded and subsequently played back, lip sync occurs in that the audio sound track would not follow its respective video images. The current professional studio practice is to cause non-phase-locked digitized video/audio material to first undergo digital to analog conversion and then analog to digital conversion at phase-locked rates to ensure relative frequency stability. The practice is costly, and time consuming. The above described practice has a further disadvantage in that it does not address the timing ill effects created by the extensive signal switching activities.
As a result, unpleasant audible `clicks` and `pops` are generated when the amount of data to be buffered exceeds the FIFO buffer size causing data to be skipped or when the amount of data in the buffer is exhausted by output causing data to be repeated. A single data skip or repeat is barely audible in the high frequency spectrum and it is considered to be acceptable by the current industry standard; but, more than one skip or repeat in series is unpleasant to the ears. FIFO buffer overflow and underflow do not occur under normal operations; however, the frequency drift and the extensive signal switching may cause data skips and data repeats. Currently, an apparatus and method to prevent multiple audio data skips and repeats occurring in series are not available for serial digital video interface.
The recent increase of video/audio switching and editing activities such as digital special effects, in professional studios has underscored the need for a low cost, improved performance, and ease of use apparatus and method addressing the video and audio synchronization timing effects brought on by a FIFO audio receiver buffer while complying with the SMPTE Standard. The present invention satisfies this need by maintaining the FIFO audio buffer at nominal fullness and delay by re-centering the buffer data size as its fullness gets out of an arbitrarily specified range.